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Maine study: Leaf cover helps ticks survive winter

Leaf litter provides significant winter protection for ticks, suggesting that leaf removal in late fall can be an effective method of reducing human exposure to tick-borne diseases such as Lyme disease the following spring, researchers in Maine have found.

“This year what we saw based at least on the first year’s data was that there was much more protection given where the leaf litter was present," Charles Lubelczyk, vector ecologist at the Maine Medical Center Research Institute’s Vector-borne Disease Laboratory, told the American Media Institute in an interview last week.

Lubelczyk’s researchers placed ticks in vials in locations throughout Cumberland County, Maine, and around New Haven, Connecticut over the winter to determine whether leaf and snow cover affected their survival rates. The light snowfall led to inconclusive results for snow cover, Lubelczyk said, but the researchers found that leaf cover created an excellent winter shelter for ticks.

“The potential for leaves to protect ticks is pretty high,” he said. When considering tick mitigation strategies at home, “one very good and easy component, and very cheap, is to remove leaves.”

This was the first year of a three-year study, meaning the results are preliminary at this point. The team plans to release a report on the early findings later this year. But the data so far strongly suggest that removing leaf litter in yards, parks and other areas where there is human activity can be an effective way to reduce tick exposure, Lubelczyk said.

“Leaves are probably, I guess you could call them kind of an overlooked area when it comes to tick management because it’s kind of boring,” he said. “It’s not high tech. It’s not going to make a lot of headlines. You’re essentially asking people for tick management to do something they don’t really like, which is to rake their leaves.”

Ticks thrive under leaf cover, said Tom Mather, who heads the Tick Encounter Resource Center at the University of Rhode Island. “Clearly if you rake your leaf litter, that’s going to remove the permissive habitat for these black-legged ticks and it will cause a little more drying out and a little less insulation in the winter.

“But that means you have to get out there and do a little more work and when you’re out there in the fall doing that, you’re exposed to ticks,” he cautioned.

Ticks require a humid environment, which leaf cover provides, said Mather, who goes by the nickname “The Tick Guy.”  They have great difficulty surviving for more than eight hours when the humidity drops below 82 percent, he said.   

“We call them a tick-averse moisture event. If ticks are held to below 82 percent humidity for eight hours or longer, they start to die, especially the nymphs," he said. “The more tick-averse moisture events that occurred from late May through June, the fewer ticks there were for the season. The fluctuation could be as much as 50 percent."

Removing leaf litter can significantly reduce tick survival rates not only in the winter, but in the spring and summer when the humidity drops for more than eight hours. The problem, Mather said, is that humidity in the eastern United States typically drops for a bit less than eight hours on drier days. The longer stretches that are needed to kill ticks do not occur frequently enough.

At his own home, Mather relies on two methods: spraying his property twice a year and wearing tick-repellant clothes, he said. 

“In your own yard, I believe that people should probably do a perimeter spray at least once a year if not twice. You only have to kill ticks once. Bisenthryn, which is the industry standard product, is quite effective and has a nice residual activity against these ticks.

“The season really is pretty important and if you were to spray by about the 15th of May, the residual activity of Bisenthryn is about four weeks, so you’re probably catching most of the ticks as they emerge.”

Spraying is “more effective I think than people would give credit for,” he said. “Now, municipalities could do the same thing if they wanted to.”

People also should treat their clothes, which provides protection wherever they go.

“The second most effective strategy, and probably it comes to the top, is just wearing tick-repellant clothes. That is a strategy that has yet to reach a tipping point. When you talk to people about it, it sounds great until they have to go about doing it.

“Before tick season starts, say February or March, pick the clothes you’re going to wear when walking your dog, doing your yard work, etc., you can send them to a company in North Carolina called Insect Shield and you can turn your own clothes into tick-repellant clothes. I think if you send in more than three items it’s about $8.30 per item. It lasts about 70 washes.

“We did a study that showed that treated socks, sneakers and shorts provided great protection. Ticks start low and crawl up. Unless you’re doing handstands all day, the place you’re likely to encounter ticks is on your lower extremity. Then you’re carrying your protection with you by wearing tick-repellant clothes.

Spraying and wearing tick-repellant clothing should be people’s first two steps, Mather said, with leaf removal an additional precaution. “All of those things are less sweat equity and are so much easier than raking the leaves away from habitat you might or might not go to. That can be an additional step, no doubt.”

In fact, “remove leaf litter” is the first recommendation on the Centers for Disease Control and Prevention’s (CDC) list of tips for how to “Create a Tick-safe Zone to Reduce Ticks in the Yard.” (http://www.cdc.gov/ticks/avoid/in_the_yard.html)

Reducing exposure to ticks is important in New England, where Lyme Disease infections have risen dramatically in the last decade. From 2005 to 2014, confirmed cases of Lyme in Maine rose from 247 in 2005 to 1,169, according to the CDC. Vermont’s cases rose from 54 to 442, and Massachusetts’ cases rose from 2,336 to 3,646.